Catalyst carrier holding material and catalytic converter

ABSTRACT

To provide a catalyst carrier holding material which is particularly useful for catalytic converters, by exhibiting heat resistance and high compression resistance in high temperature ranges, as well as excellent wind erosion resistance. The catalyst carrier holding material comprises inorganic fibers comprising alumina and silica, and the mullite ratio of the inorganic fibers is in the range of greater than 30% and less than 75%, and preferably in the range of 35% to 70%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/US02/19904 filed Jun. 21, 2002, whichclaims priority to the Japanese Patent Application No. 2001-190005 filedJun. 22, 2001, the disclosure of which is incorporated by reference inits/their entirety herein.

FIELD OF THE INVENTION

The present invention relates to a catalyst carrier holding material,and more specifically, it relates to a catalyst carrier holding materialwith excellent heat resistance, compression resistance and wind erosionresistance. The present invention also relates to a catalytic converterpacked with the catalyst carrier holding material. The catalyticconverter of the invention can be advantageously utilized for treatmentof exhaust gas from an internal combustion engine, such as that of anautomobile. The catalytic converter is preferably a catalytic converterholding a catalytic element within its casing.

BACKGROUND OF THE INVENTION

Exhaust gas purification systems employing ceramic catalytic convertersare well known as means for removing the carbon monoxide (CO),hydrocarbons (HC), nitrogen oxides (NOx) contained in exhaust gas fromautomobile engines. A ceramic catalytic converter basically comprises aceramic catalyst carrier (this is usually called the “catalyticelement”) in the shape of a honeycomb, for example, housed in a metalcasing, or housing.

As is known, there exist many different types of ceramic catalyticconverters, but the usual construction employed has the gap between thehoused catalyst carrier and the casing filled in with a heat insulatingmaterial typically composed of a combination of inorganic fibers withorganic fibers and/or a generally liquid or paste-like organic binder asdisclosed in, for example, Japanese Unexamined Patent Publication(Kokai) Nos. 57-61686, 59-10345 and 61-239100. The heat insulatingmaterial filling in the gaps thus holds the catalyst carrier, and canprevent unexpected mechanical shock due to impact or vibrations frombeing exerted on the catalyst carrier. Since the catalyst carrier is notdestroyed or shifted in a catalytic converter with this type ofconstruction, the desired operation can be realized for extendedperiods.

Incidentally, it is preferred for catalytic converters to be operated athigher temperatures in order to improve the exhaust gas purification andenhance combustion. Particularly in recent years, with the movementtoward tougher standards for exhaust gas with the aim of protecting theearth environment, there has been a trend toward more efficientpurification of exhaust gas by increasing the operating temperature. Infact, operating temperatures for catalytic converters have reached ashigh as 800-1000° C., and even higher. However, insulating materialssuch as those disclosed in the aforementioned unexamined patentpublications cannot be applied for operating temperatures in such highranges, because of their composition and others.

Attention has recently been directed toward insulating materialscomposed mainly of crystalline alumina fibers, which can withstand highoperating temperatures, and products using them have been implemented.As one example, Japanese Unexamined Patent Publication (Kokai) No.7-286514 discloses a holding material (corresponding to an insulatingmaterial) for use in an exhaust gas purification apparatus characterizedby being composed of a blanket wherein crystalline alumina fibers arearranged in layers, and sections of the fibers are oriented in thedirection normal to the layer surface by needle punching. The aluminafibers used for this holding material must have a mullite compositionwith an alumina to silica weight ratio of 70/30 to 74/26. If the aluminato silica weight ratio is outside of this range, deterioration of thefibers occurs more rapidly due to crystallization and crystal growth athigh temperature, such that the material cannot withstand use forextended periods.

Nevertheless, with the exhaust gas purification apparatus holdingmaterial described in Japanese Unexamined Patent Publication (Kokai) No.7-286514, not only are the desired function and effect exhibited onlywhen using alumina fibers having a mullite composition in theaforementioned limited range, but using alumina fibers with a lowmullite ratio (for example, a few percent) results in increased plasticdeformation and poor bearing retention (compression resistance) in hightemperature ranges. Furthermore, since the compression resistance tendsto rapidly decrease as a result of use at high temperature even if thecompression resistance at the initial stage is satisfactory, it is verydifficult to maintain high compression resistance for extended periods.On the other hand, when using alumina fibers with a high mullite ratio(for example, 75% or more), the compression resistance in hightemperature ranges is improved because of reduced plastic deformation,but the higher brittleness which also occurs results in easier breakageof the alumina fibers, such that it is impossible to avoid worsening of“wind erosion” (a phenomenon in which the holding material crumbles atboth ends due to wind pressure). As this wind erosion continues toprogress, it causes dwindling of the surface area of the holdingmaterial which is supposed to exhibit holding force, and this leads tolower overall compression resistance and thus inconveniences such asshifting of the catalyst carrier. These properties are mutuallyopposing, and it is preferred to provide an exhaust gas purificationapparatus holding material that simultaneously satisfies both propertiesof high compression resistance in high temperature ranges and itsmaintenance, together with excellent wind erosion resistance.

SUMMARY OF THE INVENTION

As explained above, various types of insulating materials have beenproposed in the past for application in catalytic converters, but allsuch insulating materials have room for improvement.

It is therefore an object of the present invention to provide a catalystcarrier holding material which exhibits heat resistance, highcompression resistance in high temperature ranges and excellent winderosion resistance, which is particularly useful in catalyticconverters.

It is another object of the invention to provide a catalytic converterpacked with such a catalyst carrier holding material for the purpose ofheat insulation, catalyst carrier holding, etc.

It is yet another object of the invention to provide a catalyticconverter which can be advantageously used for treatment of exhaust gasin the internal combustion engine of an automobile or the like.

These and other objects of the invention will become readily apparent byway of the detailed description which follows hereunder.

According to the present invention, the above problems can be solved bya catalyst carrier holding material which serves to hold a catalystcarrier, it is composed of or comprises inorganic fibers comprisingalumina and silica, and the mullite ratio of the inorganic fibers is inthe range of greater than 30% and less than 75%, and preferably in therange of 35-70%.

The inorganic fibers used in the catalyst carrier holding material canbe a blend of fibers containing less than 10% mullite and fiberscontaining greater than 70% mullite. In addition, the catalyst carrierholding material can comprise layers of the inorganic fibers, with atleast one layer comprising fibers containing less than 10% mullite andat least one other layer comprising fibers containing greater than 70%mullite.

According to the present invention, there is also provided a catalyticconverter equipped with a casing, a catalytic element situated in thecasing and a catalyst carrier holding material situated between thecasing and the catalytic element.

The catalyst carrier holding material is composed of or comprisesinorganic fibers comprising alumina and silica, and the mullite ratio ofthe inorganic fibers is in the range of greater than 30% and less than75%, and preferably in the range of 35-70%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an expanded perspective view of a preferred embodiment of acatalytic converter according to the present invention.

FIG. 2 is a graph showing a time plot of the changes in bearing at 900°C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be explained by way of its preferredembodiments. As will be readily apparent to any person skilled in theart, the invention is not limited to these embodiments, and variousmodifications and improvements may be incorporated therein within thespirit of the invention.

The catalytic converter according to the present invention has aconstruction comprising at least a casing and a catalytic elementsituated in the casing, with a catalyst carrier holding materialaccording to the invention described in detail hereunder packed betweenthe casing and the catalytic element, in such a manner as to surroundthe catalytic element. Depending on the purpose of use, the catalystcarrier holding material may loaded in a section of the catalyticelement, or loaded in the entirety thereof. If necessary, attachingmeans such as wire mesh may be used as supplementary means. The catalystcarrier holding material is preferably used with appropriatecompression, so that it has the proper density when loaded into thecasing. Compression methods include clamshell compression, stuffingcompression, tourniquet compression and the like.

The catalytic converter of the present invention encompasses many typesof catalytic converters, but it is preferably a catalytic converterprovided with a monolithically molded catalytic element, i.e., amonolithic catalytic converter. This type of catalytic convertercomprises a catalytic element with a honeycomb-shaped cross-sectionhaving small channels and is therefore smaller than conventionalpellet-type catalytic converters, and the contact area with the exhaustgas can be sufficiently ensured while also minimizing the exhaustresistance; as a result, it is possible to treat exhaust gas in a moreefficient manner.

The catalytic converter of the present invention may be advantageouslyused for treatment of exhaust gas when combined with various types ofinternal combustion engines. The excellent function and effect areexhibited in sufficient fashion when the catalytic converter of theinvention is mounted in the exhaust system of an automobile such as apassenger vehicle, bus, truck or the like.

FIG. 1 is a perspective view showing a typical example of a catalyticconverter according to the present invention, and it shows an expandedview of the catalytic converter, for easier explanation of theconstruction. The illustrated catalytic converter 10 is provided with ametal casing 11, a monolithic solid catalytic element 20 situated insidethe metal casing 11, and a catalyst carrier holding material 30 situatedbetween the metal casing 11 and the catalytic element 20. The catalystcarrier holding material 30 is constructed of inorganic fibers composedof or comprising alumina and silica, and the mullite ratio of theinorganic fibers is in the range of greater than 30% and less than 75%,and preferably in the range of 35-70%, as according to the invention. Aswill be explained in detail below, the catalyst carrier holding material30 has a coating 31 to protect the surface from damage. The coating 31may of course be omitted if it is not necessary. An exhaust gas inlet 12and exhaust gas outlet 13 each having a truncated cone shape areincorporated in the shape of the catalytic converter 10.

As a more specific explanation, the solid catalytic element in the metalcasing comprises a ceramic catalyst carrier with a honeycomb structurehaving a plurality of exhaust gas channels (not shown). The catalystcarrier holding material of the invention is placed so that it envelopsthe catalytic element. In addition to functioning as an insulatingmaterial, the catalyst carrier holding material also holds the catalyticelement inside the metal casing and seals the gap created between thecatalytic element and the metal casing, either to prevent the exhaustgas from flowing around the catalytic element or at least to minimizesuch undesirable flow. The catalytic element is supported firmly andelastically inside the metal casing.

The metal casing in the catalytic converter of the present invention maybe fabricated from any of various metal materials publicly known in thefield, into any desired shape depending on the intended function andeffect. A suitable metal casing is made of stainless steel in the shapeshown in FIG. 1. If necessary, of course, the metal casing may befabricated into any desired appropriate shape from a metal such asaluminum or titanium, or an alloy thereof.

Like the metal casing, the solid catalytic element will usually befabricated from the same manner of material used for common catalyticconverters, and in a similar shape. Suitable catalytic elements whichare well known in the field include those manufactured from metal,ceramic and the like. A useful catalytic element is disclosed, forexample, in U.S. Reissued Pat. No. 27,747. Ceramic catalytic elementsare commercially available from Corning Inc., USA, and elsewhere. Forexample, honeycomb-shaped ceramic catalyst carriers are commerciallyavailable from Corning Inc. under the trade name “CELCOR”, and from NGKInsulated Ltd. under the trade name “HONEYCERAM”. Metal catalyticelements are commercially available from Behr GmbH and Co., Germany. Fora detailed description regarding monolithic catalysts, refer to, forexample, “System Approach to Packaging Design for Automotive CatalyticConverters” by Stroom et al., SAE Technical Papers, Document No. 900500;“Thin Wall Ceramics as Monolithic Catalyst Support” by Howitt et al.,SAE Technical Papers, Document No. 800082; and “Flow Effect inMonolithic Honeycomb Automotive Catalytic Converter” by Howitt et al.,SAE Technical Papers, Document No. 740244.

Catalysts to be carried in the aforementioned catalytic elements willusually be metals such as platinum, ruthenium, osmium, rhodium, iridium,nickel, palladium, etc. and metal oxides such as vanadium pentaoxide,titanium dioxide, etc., and they are preferably coated for use. Adetailed explanation on coating of such catalysts may be found in U.S.Pat. No. 3,441,381.

In the practice of the present invention, the catalytic converter may befabricated with any desired construction and method, provided that it iswithin the scope of the invention. The catalytic converter is basicallyfabricated by housing in a metal casing a honeycomb-shaped ceramiccatalyst carrier (catalytic element), for example, and it isparticularly preferred to fabricate the catalytic element by carrying acatalyst layer (catalyst coating) comprising a precious metal such asplatinum, rhodium or palladium on a honeycomb-shaped ceramic monolith,for example. Employing such a construction can exhibit an effectivecatalytic function at relatively high temperatures.

According to the present invention, the catalyst carrier holdingmaterial of the invention is situated between the metal casing and thecatalytic element inside it. The catalyst carrier holding material maybe constructed of a single member, or it may be constructed of two ormore members in a laminated or bonded fashion. It will usually beadvantageous from the standpoint of manageability for the catalystcarrier holding material to be in the shape of a mat, blanket or thelike. The size of the catalyst carrier holding material, of course, maybe varied within a wide range depending on the purpose of use. Forexample, when a mat-shaped catalyst carrier holding material is to beused to fill an automotive catalytic converter, the holding materialwill normally have a mat thickness of approximately 1.5-10.0 mm, a widthof approximately 100-1000 mm and a length of approximately 200-1500 mm.Such a holding material may, if necessary, be cut into the desired shapeand size with scissors, a cutter or the like.

The inorganic fibers composing the catalyst carrier holding materialconsist of inorganic fibers containing alumina (Al₂O₃) and silica(SiO₂), as mentioned above. The inorganic fibers comprise these twocomponents of alumina and silica, preferably with an alumina/silicamixing ratio in the range of 50:50 to 80:20. If the alumina/silicamixing ratio is outside of this range, for example if the alumina mixingratio is less than 50%, an undesirable situation of poor heat resistancemay result.

The proportion of mullite (3Al₂O₃.2SiO₂) present in these inorganicfibers, i.e. the mullite ratio, is in the range of greater than 30% andless than 75%, and preferably in the range of 35-70%, more preferably inthe range of 60-70%, and most preferably around 65%. According to thepresent invention, setting the mullite ratio of the inorganic fibers ofthe catalyst carrier holding material within the range of greater than30% and less than 75%, and preferably in the range of 35-70%, can givean improved holding material with relatively high initial bearing, whichsimultaneously allows minimized bearing reduction and prevents loweredwind erosion resistance. Incidentally, if the mullite ratio is too muchbelow 35%, the initial bearing is high but increased plastic deformationreduces the bearing retention in high temperature ranges. Conversely, ifthe mullite ratio is too much higher than 70%, the bearing retentionreduction can be minimized since increasing plastic deformation isprevented, but the higher mullite ratio also leads to greaterbrittleness, thus leading to greater brittleness of the fibersthemselves. As the inorganic fibers become more brittle, the fibers tendto break up more finely during molding of the mat, and this results in aloss of wind erosion resistance which is another of the importantproperties for use of the catalyst carrier holding material, as well aslower initial bearing.

Described in detail, the mullite ratio of the inorganic fibers, if it isreferred to in the specification of this application, can define as aratio of peak strength determined in accordance with the followingprocedure.

A sample of the inorganic fibers for which a mullite ratio should bedetermined is introduced in a X-ray diffractometric apparatus todetermine a peak strength “A” at a diffraction angle of 26.3°. Then, theinorganic fibers having the same alumina/silica ratio and averagediameter as those of the sample are heated at 1500° C. for 8 hours toprepare a standard sample. The standard sample is introduced in a X-raydiffractometric apparatus to determine a peak strength “A₀” at adiffraction angle of 26.3°. The mullite ratio of the inorganic fibers,tested as the sample, is defined as a percentage of A/A₀.

For the reference, the X-ray diffractometric apparatus used herein is“RINT 1200” (trade name) commercially available from Rigaku DenkiKabushikikaisha, and the determination was carried out under thefollowing conditions.

Tube Voltage: 40 kV

Tube Current: 30 mA

Target: Cu

Angle: 20-40°

Scan speed: 2°/min.

Step sampling: 0.02°

Slit (RS): 0.3 mm

Smoothing Point: 13

The inorganic fibers may be used alone, or combinations of two or moretypes of fibers may be used. In particular, using a mixture of two ormore different types of inorganic fibers with different mullite ratioscan more easily achieve the effect described above, while also allowingfine adjustment of the effect. For example, reduced bearing can becompensated by inorganic fibers with a high mullite ratio, while reducedwind erosion resistance can be compensated by inorganic fibers with alow mullite ratio.

The thickness (mean diameter) of the inorganic fibers is notparticularly restricted, but the fibers preferably have a mean diameterof 2-7 μm. If the inorganic fibers have a mean diameter smaller than 2μm, they tend to be brittle with insufficient strength, whereas if theyhave a mean diameter of larger than 7 μm, they tend to be less suitablefor molding into a holding material.

As with the thickness, there are likewise no particular restrictions onthe lengths of the inorganic fibers. However, the inorganic fiberspreferably have an average length of 0.5-50 mm. If the inorganic fiberlength is smaller than 0.5 mm, the effect of the holding material formedusing those fibers may not be exhibited, whereas if it is larger than 50mm the fibers become difficult to manage, thus complicating smoothprogress in the holding material manufacturing process.

The catalyst carrier holding material of the present invention may alsocontain other additional components with the aforementioned inorganicfibers, for integration of the inorganic fibers or for other purposes.Suitable additional components include, but are not limited to,integrating components, flocculating agents, heat expanding agents, etc.

The catalyst carrier holding material of the present invention may bemanufactured by any of various well-known and commonly employed methods.Such manufacturing methods can include wet methods, or dry methodsutilizing a needle punch or the like. According to the invention, wetmethods may be used with particular advantages. Such methods involvesimpler manufacturing processes and do not require large-scalemanufacturing equipment. As an example, the catalyst carrier holdingmaterial of the invention may be advantageously manufactured by thefollowing wet method.

First, the inorganic fibers and binder are placed in water, for openingand mixing of the fibers. Next, the mixture is stirred while adding aninorganic or organic flocculating agent thereto to prepare a slurry. Theslurry is then sheeted and molded into the desired shape. The thusobtained molded product is squeezed to remove the excess moisture. Next,the molded product is pressed while heating at the desired temperaturefor drying to obtain a holding material. This operation may be carriedout, for example, by placing the molded product into an oven and heatingand drying at 150° C. for 20 minutes. If necessary, the holding materialmay be made into a laminated product.

Following this, at least one side of the dried holding material iscoated with an appropriate coating material by a well known commonlyemployed technique such as spraying or coating, and optionally laminatedwith tape. In most cases, the coating material will be diluted 2- to10-fold with a solvent (for example, water or an organic solvent) andthen coated onto at least one side, preferably both the front and backsides, of the holding material. The desired holding material may beobtained in this manner.

EXAMPLES

The present invention will now be explained by way of examples. It is tobe understood, however, that the invention is in no way limited to thesespecific examples.

Example 1

Manufacture of Catalyst Carrier Holding Materials (Mats)

The following seven different mat-shaped catalyst carrier holdingmaterials were manufactured using inorganic fibers composed of orcomprising the two components alumina and silica, having analumina/silica mixing ratio of 72:28 and different mullite ratios (2%,35%, 60%, 70% and 78%).

For manufacture of each of the holding materials, first the inorganicfibers (a mixture of two different inorganic fibers for manufacture ofholding material G) and the organic binder (trade name: “LX-816”)(flocculating agent) were placed in water, and the fibers were openedand mixed. Next, the mixture was gently stirred to prepare a slurry. Theslurry was then sheeted and molded into the desired mat shape. Themolded product was squeezed to remove the excess moisture, and then themat was pressed while heating to dryness at the prescribed temperature.This yielded the following catalyst carrier holding materials eachhaving a thickness of 8 mm, a width of 250 mm and a length of 250 mm.

Holding material A . . . 2% mullite ratio of inorganic fibers

Holding material B . . . 35% mullite ratio of inorganic fibers

Holding material C . . . 60% mullite ratio of inorganic fibers

Holding material D . . . 65% mullite ratio of inorganic fibers

Holding material E . . . 70% mullite ratio of inorganic fibers

Holding material F . . . 78% mullite ratio of inorganic fibers

Holding material G . . . Mixture of inorganic fibers with 2% mulliteratio and inorganic fibers with 78% mullite ratio (50/50 mixing ratio)

Evaluation of Properties of Catalyst Carrier Holding Materials

The bearing retention (compression resistance) and wind erosionresistance of the catalyst carrier holding materials A-G manufactured inExample 1 were measured according to the following procedure.

Measurement of Bearing Retention:

The mat-shaped holding material was cut into a 45 mm diameter disk, andits weight was measured. Next, the disk-shaped holding material samplewas sandwiched between two stainless steel plates, and pressed in thedirection of its thickness to a packing density of 0.3 g/cm³ (ignoringthe loss of organic components, etc. by firing). Next, the holdingmaterial sample was heated to 900° C. and the bearing at that time wasmeasured. In order to determine the change in bearing with time, thebearing measurement was continued for 20 hours, once every hour, wheretime 0 was the point of initial measurement. The change in bearing canbe approximated using the formula Y=aX^(b) (where X is the compressionforce), and b was the slope of the change in bearing. The evaluation wasbased on the slope of the curve of measured change with time at 900° C.,with acceptable samples evaluated or judged as O and unacceptablesamples evaluated or judged as x.

Measurement of Wind Erosion Resistance:

The mat-shaped holding material was cut into a flat sample with a widthof 25 mm and a length of 50 mm, and the weight was measured. Next, theflat holding material sample was sandwiched between two stainless steelplates, and pressed in the direction of its thickness to a packingdensity of 0.2 g/cm³ (ignoring the loss of organic components, etc. byfiring). While heating one of the plates to 800° C. and the other plateto 600° C., compressed air at 90 kPa heated to 600° C. was blown ontoone end of the holding material sample (in the widthwise direction) fora period of 2 hours. Upon completion of the compressed air blowing, thewind erosion per unit time was determined. The evaluation was based onthe measured wind erosion, with acceptable samples evaluated as O andunacceptable samples evaluated as x.

Table 1 below summarizes the results of the two different evaluationtests described above. FIG. 2 is a graph showing a time plot of thechanges in bearing at 900° C. for holding materials A, D, F and G, forreference. The bearing change formula: Y=aX^(b) for each of theseholding materials was as follows.

Holding Material A . . . Y=_(103.58)X^(−0.088)

Holding Material D . . . Y=₈₃₅₈₇X⁻0.0368

Holding Material F . . . Y=_(75.842)X^(−0.0213)

Holding Material G . . . Y=_(91.979)X^(−0.0548)

TABLE 1 Slope of time change Eval- Wind Eval- Extrapolated Type ofholding curve at uation erosion uation to 10 years material 900° C.(Judge) (g/hr) (Judge) later Holding material A −0.088 X 0.01 ◯ 38.1Holding material B −0.0605 ◯ 0.02 ◯ 45.0 Holding material C −0.0428 ◯0.02 ◯ 53.8 Holding material D −0.0368 ◯ 0.03 ◯ 61.4 Holding material E−0.0387 ◯ 0.03 ◯ 82.1 Holding material F −0.0213 ◯ 0.10 X 59.5 Holdingmaterial G −0.0548 ◯ 0.02 ◯ 49.3

In this test, the pressure change with time at 900° C. was taken as theindex for evaluating the durability of the ability to hold the catalystcarrier (holding force). A 10 year extrapolation with the pressurereduction approximating formula is usually used to determine thepressure necessary to stably hold a catalyst carrier (compressionforce). It is desirable for this pressure to be at least 35 kPa,preferably 45 kPa or greater, and more preferably 50 kPa or greater.When the evaluation results are considered from this viewpoint, theproper compression force is obtained when the mullite ratio is greaterthan 30%, preferably 35% or greater, and more preferably 60% or greater,and less than 78%.

Example 2

The desirable properties experienced with the present invention can beobtained from a mat made by blending low mullite content fiber (e.g.,less than 10%) and high mullite content fiber (e.g., greater than 70%).Such properties can also be obtained, for example, with a layered mat,using low mullite content fibers for at least one layer and high mullitecontent fibers for at least one other layer.

Sample mat-shaped catalyst carrier holding materials were made asdescribed for Example 1 above with 2% and 78% mullite content fibers.Fiber blend ratios and layering ratios were figured as average mullite %calculated by fiber weight and its fiber % mullite. For example, theaverage mullite % of a mat made of a 50/50 blend of these mullite fibers(i.e., 2% and 78% mullite content fibers) is 40%. Compression at 900deg.C. and erosion resistance were evaluated and results were summarizedbelow in Table 2.

Inclination is b in approximated equation: Y=aX^(b)

TABLE 2 Inclination Extrapolated Ero- Average of Compression sion TotalMullite % Compression after 10 yrs Judge (g/hr) Judge Judge Blend  2%−0.088 38.1 NG 0.01 Pass NG 20% −0.082 34.9 NG 0.01 Pass NG 30% −0.06843.1 NG 0.02 Pass NG 35% −0.064 46.5 Pass 0.01 Pass Pass 40% −0.054 49.3Pass 0.02 Pass Pass 60% −0.051 52.1 Pass 0.02 Pass Pass 70% −0.037 62.0Pass 0.03 Pass Pass 78% −0.021 59.5 Pass 0.10 NG NG Layered  2% −0.08838.1 NG 0.01 Pass NG 20% −0.076 39.2 NG 0.02 Pass NG 30% −0.080 40.2 NG0.01 Pass NG 35% −0.066 45.7 Pass 0.02 Pass Pass 40% −0.049 48.2 Pass0.02 Pass Pass 60% −0.035 55.4 Pass 0.02 Pass Pass 70% −0.039 59.0 Pass0.04 Pass Pass 78% −0.021 59.5 Pass 0.10 NG NG 65% 30% −0.071 41.0 NG0.01 Pass NG Mullite Fiber

As explained above, according to the present invention, there isprovided a catalyst carrier holding material which is particularlyuseful for catalytic converters, by exhibiting heat resistance and highcompression resistance in high temperature ranges, as well as excellentwind erosion resistance.

There is further provided according to the present invention a catalyticconverter packed with a catalyst carrier holding material of the presentinvention for the purpose of heat insulation, catalyst carrier holdingand the like.

There is still further provided according to the present invention acatalytic converter that can be advantageously utilized for treatment ofexhaust gas from an internal combustion engine, such as that of anautomobile.

1. A catalyst carrier holding material comprising inorganic fiberscomprising alumina and silica, said inorganic fibers having an averagemullite percent greater than 30% and less than 75%.
 2. A catalystcarrier holding material according to claim 1, wherein said inorganicfibers have an alumina to silica mixing ratio in the range of 50:50 to80:20.
 3. A catalytic converter comprising a casing, a catalytic elementsituated in said casing and a catalyst carrier holding materialaccording to claim 1 being situated between said casing and saidcatalytic element.
 4. A catalytic converter according to claim 3,wherein said inorganic fibers comprise the two components of alumina andsilica, and the mixing proportion of the alumina and silica is in therange of 50:50 to 80:20.
 5. A catalytic converter according to claim 3,wherein said catalytic element is a monolithic catalytic element.
 6. Acatalytic converter according to claim 3, wherein said inorganic fibersare a blend of fibers containing less than 10% mullite and fiberscontaining greater than 70% mullite.
 7. A catalytic convener accordingto claim 3, wherein said holding material comprises layers of saidinorganic fibers, with fibers in at least one layer contatning less than10% mullite and fibers in at least one other layer containing greaterthan 70% mullite.
 8. A catalyst carrier holding material according toclaim 1, wherein said inorganic fibers have an average in mullitepercent in the range of from 60% to 70%.
 9. A catalyst carrier holdingmaterial comprising inorganic fibers comprising alumina and silica, saidinorganic fibers being a blend of (a) inorganic fibers containing lessthan 10% mullite and (b) inorganic fibers containing greater than 70%mullite, wherein said blend has an average mullite percent in the rangeof from greater than 30% to less than 75%.
 10. A catalytic convertercomprising a casing, a catalytic element situated in said casing and acatalyst carrier holding material according to claim 9 being situatedbetween said casing and said catalytic element.
 11. A catalyst carrierholding material comprising multiple layers of inorganic fiberscomprising alumina and silica, said inorganic fibers in at least onelayer containing less than 10% mullite and said inorganic fibers in atleast one other layer containing greater than 70% mullite, wherein saidmultiple layers have an average mullite percent in the range of fromgreater than 30% to less than 75%.
 12. A catalytic converter comprisinga casing, a catalytic element situated in said casing and a catalystcarrier holding material according to claim 11 being situated betweensaid casing and said catalytic element.
 13. A catalytic converteraccording to claim 12, wherein said inorganic fibers comprise the twocomponents of alumina and silica, and the mixing proportion of thealumina and silica is in the range of 50:50 to 80:20.
 14. A catalyticconverter according to claim 12, wherein said catalytic element is amonolithic catalytic element.
 15. A catalytic converter according toclaim 14 wherein said inorganic fibers comprise the two components ofalumina and silica, and the mixing proportion of the alumina and silicais in the range of 50:50 to 80:20.
 16. A catalytic convertcr accordingto claim 14, wherein said catalytic element is a monolithic catalyticclement.